My friend Pat, ZL1AXB has come up with a unique tuned filament
input circuit that allows near full power output from 813 tubes
all the way up to 10 meters. He has achieved 1000 watts out from
two tubes on 10 which is a feat that many hams will tell you is
impossible. In the spring of 1998, he talked me into building my
own amp using his design. Chuck, WA7RAI emailed this schematic of
the amp, which I mostly followed with a few modifications of my
own:

It is basically your standard pentode in grounded grid with a
PI output network. The secret weapon lies in the input circuit.
Instead of a filament choke, Pat uses a 1 inch Diameter coil of
¼" copper tubing that has an insulated wire inside. This
coil is connected in parallel with a variable capacitor and
swamped with a non-inductive resistor. Filament voltage is
applied through the insulated wire and the coil. The exciter
power is injected through a .01 cap to the coil. This "tuned
filament" circuit presents a low to flat SWR to the
transmitter on any band in the tuning range of the circuit making
it ideal for modern transistorized finals with high SWR auto
power reduction.

This is my version of Pats amp:

I use a switchable input coil that can tune 160 to 10 by
switching in more or less capacitor and coil. One setting for 40
to 160 and another for 30 to 10. I also use a roller inductor
instead of a taped coil in the tank circuit because my Elmer,
Harry Longrich W4ANL gave it to me from his parts pile along with
many other components you see above. The case was made from heavy
gauge sheet metal salvaged from an underground transformer cover.
All of the power supply components are located in a box in my
closet so I dont hear the hum and fan noise. I made lots of
smoke getting it to work and must thank Rod ZL2ACE, Chuck, WA7RAI,
and Pat ZL1AXB for encouragement and long distance trouble
shooting.

With 3000 volts on the plate from a 2 kva pole pig. The two
813s produce over 1 KW out on 80 to 20 meters, 900W on 17
and 15, 800W on 12 and 500W on 10 meters. My wide open layout and
the long lead lengths limit the output on 10, meters and I dont
have enough coil or capacitor in the tank circuit for 160 to work
at all. Since I seldom use 160 and 500 W is big QRO on 10, I am
very satisfied with the performance.